In EP-B-O 105 980 a heat exchange apparatus for use in a FCC process is described which comprises a housing of vertical orientation in which in the upper part an inlet opening for solid particles is provided and in which near the bottom fluidization means are arranged. The inlet opening communicates with a reservoir of solid particles to be cooled, in particular a fluidized bed of fluid catalytic cracking catalyst. Heat exchange tubes are provided at least in the space defined between the fluidization means and the inlet opening. According to this reference there are two ways for the cooled solid particles to be withdrawn. A first embodiment provides the withdrawal of solid particles at the bottom of the housing via conduit and the cooled solid particles are subsequently re-introduced into the reservoir of solid particles by means of a standpipe and riser system. In a second embodiment cooled solid particles are withdrawn via the inlet opening. This is accomplished by the creation of a backmixing zone in the housing by means of the use of the fluidization gas entering the housing via the fluidization means.
The second embodiment eliminates the necessity to use expensive conduits, standpipes, riser pipes and auxiliary means such as valves and expansion joints. However, the mass transfer in this second embodiment is slower than in the first. Hence, the duty of the heat exchange tubes is worse. Moreover, it will be clear that mass transfer of cooled solid particles from the housing to the reservoir and hence the heat exchange duty of the apparatus will be less controllable than in the first embodiment.